Waste material that is disposed of in landfills is typically collected in one location until a sufficient amount has been accumulated and covered with fill material. Once a sufficient amount of waste material has been accumulated, the waste material is distributed and covered with soil. Heavy machinery such as bulldozers and the like are used to distribute the soil over the waste material, or to construct a pit to accommodate the waste material. Whichever method is used the result is the same, the waste material is buried with soil and compacted. After the waste material is buried, the process of collecting, burying, and compacting the waste material repeats itself. As a result, it is not uncommon for a landfill to comprise several hundred feet of buried waste material.
Waste materials buried in the landfill that are biodegradable will eventually undergo a decomposition reaction, producing gases which are high in methane and quite flammable. As the waste material continues to decompose, the production rate of the gases increases, increasing the pressure of the gases. Unless the gases are collected below the landfill surface, the rising pressure of the gases may cause the gases to find their way to the surface. If allowed to permeate the landfill surface, the gases will create both an air pollution problem and a explosion hazard that may be ignited by any nearby source, such as a garbage truck, bulldozer or the like. After the gases have been allowed to travel to the landfill surface they are extremely difficult to contain due to their inherent ability to migrate horizontally through the soil to a different surface location.
In order to protect against such dangers, wells can be constructed in the landfill to collect the gases below the landfill surface before they are allowed to migrate to the landfill surface. These wells are constructed by first boring a hole into the surface of the landfill and down through the various layers of fill and waste material to a level where gas generation is detected. Sections of pipe are lowered down into the hole and connected together to form a continuous pipe string which extends from the surface throughout the depth of the hole. The types of pipe used in such wells typically have a perforated surface area near the lower end to permit the passage of the gases from the surrounding fill material into the pipe and upward to the landfill surface for collection. The gases are extracted from the waste material surrounding the perforated pipe surface of the pipe string. The upper portion of the pipe has a solid wall. The pipe string is subjected to a vacuum at the surface to facilitate the efficient extraction of the gases.
Perforated pipes that are used to extract gases from landfill wells are known in the art. These pipes may be constructed from existing non-perforated pipe by simply forming holes in the surface of the pipe. These pipes may, therefore be made from well known pipe materials such as carbon steel and the like. However, the down-hole conditions in landfill wells are extremely corrosive making the use of carbon steel pipes impractical.
Perforated pipes are known to be constructed from polyethylene. The use of polyethylene as a pipe material eliminates the problem of corrosion. However, the decomposition reaction of waste material is exothermic, oftentimes generating down-hole temperatures in the range of from 150.degree. to 180.degree. F. There are forces tending to crush the pipe. Additionally, the pipe sections in a pipe string are oftentimes subjected to down-hole lateral forces caused by shifting waste material. Therefore, in order to ensure that a pipe made from polyethylene will have a sufficient degree of radial stiffness to resist deformation and collapse when subjected to such temperatures and lateral forces, a polyethylene pipe must be manufactured having a continuous wall thickness of significant dimension. For example, a polyethylene pipe having a twelve inch diameter for use in a landfill well is constructed having at least a two inch wall thickness in order to survive the down-hole temperatures, crushing loads and lateral forces.
The need to construct a polyethylene pipe having such a significant wall thickness functions to increase both the diameter of the bore hole that must be created to accommodate the pipe, and the weight of each pipe section used to construct the pipe string. Increasing each of these factors effectively increases the time, cost, and effort required to both manufacture the pipe and construct a well using such pipe. These factors are of considerable importance considering the fact that a single landfill may construct and operate as many as 50 wells up to 200 feet deep, or more in larger landfills.
Additionally, it is well known that the depth of compacted waste material and soil in a landfill is unstable and oftentimes undergoes a significant settling as the waste material decomposes and is converted to gas. Such settling is not uniform across the landfill and lateral shifting often occurs. Pipe strings in landfill wells are often subjected to lateral shear forces imposed by surrounding layers of waste materials that have shifted. Pipes manufactured from carbon steel have been known to break when subjected to these shear stresses. Pipes manufactured from polyethylene have been known to collapse when subjected to similar shear forces. A pipe string comprising a broken or collapsed pipe section effectively renders the respective landfill well useless because the vacuum that is pulled on the pipe string no longer results in the extraction of the gases from the waste material surrounding the entire pipe string, but rather only effects a localized extraction of gases from the discrete section of waste material residing above the broken or collapsed portion.
It is, therefore, desirable to construct a perforated pipe that is capable of withstanding the down-hole corrosive conditions in a landfill well without loss of mechanical integrity. It is also desirable to construct a perforated pipe that is capable of withstanding the down-hole temperature and shear conditions in a landfill well without requiring a significant wall thickness that will facilitate the economic and efficient construction of both the pipe and landfill wells incorporating such pipes.
It is further desirable to construct a perforated pipe that is capable of being joined together with other pipes in a manner that will accommodate the lateral forces exerted upon the pipe string, so as to minimize the effects of such forces upon each individual pipe section. It is also desirable that the pipe be constructed in an economic manner using practical manufacturing methods.